Welcome to the DIS boards, David!
If you book a cabin for 4 people,
DCL will put you into one of the cabins that sleeps 4 people. It's as simple as that
There are 2 twin beds that can be joined together to make a queen bed. Then the sofa folds down into a twin bed, and for the 4th passenger, a bunk type of bed folds down out of the ceiling, over the sofa.
In your case, with your kids so young, I'm not sure you'd want either of your children up in that bunk. I believe they have those folding play pen types things for babies and toddlers to sleep in. In that case, I'm wondering if they'd put you in a cabin that normally would sleep 3 people, PLUS give you the crib thing for your youngest child.
I'm sure others will be along and be able to help you a little more with this exact info.
